A position measuring system of this type is used particularly in processing machines for measuring the relative positions of a tool with respect to a workpiece to be processed.
European Patent Publication EP-0 151 002 B1 describes a magnetic position measuring system for measuring the relative position of two objects which are movable with respect to each other. A periodic graduation is scanned by a scanning unit having two groups of magneto-resistive elements, each group having four magneto-resistive elements, for generating zero-symmetrical output signals as shown in FIG. 19. The magneto-resistive elements of the two groups having output signals with phase relations of 0.degree. and 180.degree. are connected in series to form a first half-bridge circuit and the magneto-resistive elements of the two groups having output signals with phase relations of 90.degree. and 270.degree. are connected in series to form a second half-bridge circuit. Two zero-symmetrical output signals with a mutual phase shift of 90.degree. are present at the center taps of the two half-bridge circuits for obtaining position measurement values in a downstream evaluation device. This position measuring system has the disadvantage that the physical layout of the magneto-resistive elements requires the connecting strip conductors of the magneto-resistive elements to repeatedly cross over each other thereby making it necessary to provide an insulating layer between the crossing connecting strip conductors. The required tempering of the insulating layer causes an increase in the hysteresis of the two zero-symmetrical output signals. This additional provision of the insulating layer and the cross-over layer makes the scanning unit more expensive and complicated to manufacture and can lead to an increase of rejected devices during manufacture.
A similar magnetic position measuring system is described in German Patent DE-PS 37 19 328 wherein a periodic graduation is scanned by a scanning unit having four groups of magneto-resistive elements, each group having four magneto-resistive elements, for generating zero-symmetrical output signals as shown in FIG. 2a. To avoid crossing of the connecting strip conductors, the four magneto-resistive elements of each group are no longer disposed together within one index period of the graduation. Instead, there is a distance in the direction of measurement of more than five index periods between the magneto-resistive element with the output signal of the phase relation of 0.degree. and the magneto-resistive element with the output signal of the phase relation of 180.degree. (push-pull elements), as well as between the magneto-resistive element with the output signal of the phase relation of 90.degree. and the magneto-resistive element with the output signal of the phase relation of 270.degree. (push-pull elements) of the same group. It is therefore only possible to filter out interference effects which extend over more than five index periods, because in this case the interference effects influence the related push-pull elements in the same way. On the other hand, short-wave interference effects of less than five index periods have a disadvantageous effect on the accuracy of measurement.